In electrophotographic applications such as xerography, a charge retentive surface is electrostatically charged, and exposed to a light pattern of an original image to be reproduced to selectively discharge the surface in accordance therewith. The resulting pattern of charged and discharged areas on that surface form an electrostatic charge pattern (an electrostatic latent image) conforming to the original image. The latent image is developed by contacting it with a finely divided electrostatically attractable powder referred to as "toner". Toner is held on the image areas by the electrostatic charge on the surface. Thus, a toner image is produced in conformity with a light image of the original being reproduced. The toner image may then be transferred to a substrate or support member (e.g., paper), and the image affixed thereto to form a permanent record of the image to be reproduced. Subsequent to development, excess toner left on the charge retentive surface is cleaned from the surface. The process is well known, and useful for light lens copying from an original, and printing applications from electronically generated or stored originals, where a charged surface may be imagewise discharged in a variety of ways. Imaging systems using imagewise ion projection to a charge retentive surface to form an electrostatic latent image developable with toner operate similarly.
Developing material commonly used in systems for developing latent images on the charge retentive surface typically comprises a mixture of toner and a "carrier" of larger granular beads of a magnetic material. If the developing system is a magnetic brush assembly, magnetizable carrier beads also provide mechanical control for the formation of magnetic brush bristles so that toner can readily be brought into contact with the charge retentive surface. Toner is attracted to the latent image from the carrier beads to form the toner image. In this type of system, some carrier beads inevitably adhere to the charge retentive surface after the image is developed. These adhering carrier beads prevent intimate contact between the substrate and toner image during the transfer step. It is thus essential for optimum operation that carrier beads remaining on the charge retentive surface be removed therefrom. Failure to remove carrier beads from the charge retentive surface results in a characteristic copy quality defect displaying a white area with a black dot in the center within a black area. The hard carrier beads also have tendency to abrade and damage the charge retentive surface if not removed prior to reaching the cleaning zone. The carrier beads also pose a threat of damage to cleaning devices.
Carrier bead removal devices are known, such as for example, U.S. Pat. No. 3,894,513 to Stanley et al. and U.S. Pat. No. 3,834,804 to Bhagat et al., which use a stationary magnet having a cylindrical shell rotating thereabout to remove the ferrous carrier beads from the photoreceptor for deposit in a sump or for return to the developer housing. Other bead pickoff devices are known, such as, for example, U.S. Pat. No. 4,210,397 to Macaluso et al. which suggests the use of an electromagnetic bead collector which is periodically activated for the collection of carrier beads on a non-magnetizable surface, and inactivated for the release of the beads along a return path to the developer housing. However, an electromagnetic bead collector is relatively expensive, costly to implement, and requires a rather large current source. U.S. Pat. No. 4,190,351 to Macaluso et al. shows a bead removal arrangement in which the carrier beads are removed from the photoconductive surface by means of a movable magnet and a fixed nonmagnetizable shield mounted in close association between said magnet and the photoconductive surface. During the copying cycle, the magnet is moved adjacent the fixed shield to cause magnetizable articles to be drawn against the shield from the photoconductive surface. After the copying cycle, the magnet is moved away from the fixed shield to withdraw the strong magnetic field from the shield, causing the magnetizable particles to fall from the shield into a collection tray by means of gravity. Magnetic arrangements are known for the removal of magnetic material from a surface, including U.S. Pat. No. 4,552,451 to Yamazaki et al. and JP-A No. 59-94776 to Iwamasa.
It would be highly desirable to simply provide a magnetic member closely associated with the charge retentive surface for the removal of carrier beads therefrom, avoiding the need for moving parts or complex controls to operate a bead removal arrangement. However, it will no doubt be appreciated that over time, carrier beads would accumulate at such a magnetic member, and, unless removed, could cause damage or undesired abrasion of the charge retentive surface. Additionally, it is quite expensive to provide a dedicated bead removal arrangement.
A recent trend towards providing automatically actuable second color development has been noted, in which, for second color development, a second developer housing is provided. Each developer housing in the machine may be automatically brought into and out of operational developing position with respect to the charge retentive surface. The movement of the developer housings into and out of developing position allows selective color development of a portion or the entire of the electrostatic latent image. With movement of the second housing into developing position, the first housing is moved out of position, and vice versa.